The present invention relates to a coating composition and a method for forming a metal oxide thin film on a substrate using the coating composition. The film-forming method belongs to the sol-gel process. In this method, a coating composition (sol) is applied to a substrate with a reverse roller coater so as to form a thin film on the substrate, and then the thin film is baked so as to transform the thin film into an optical film (metal oxide film).
There are various proposed methods for applying a coating composition to a substrate, as follows. In a dipping method, a substrate is dipped into a coating composition and then the substrate is withdrawn therefrom at a certain speed. In a flow coating method, a coating composition is flowed over a substrate. In a direct roller coating method, a coating composition is applied to a rotating plastic roller, and then this wet roller is rolled on a substrate in a direction which is the same as the moving direction of the substrate while the substrate is moved below the roller. In a spin coating, a coating composition is dropped to a substrate and the dropped coating composition is spread on the substrate by rotating the substrate.
According to dipping and spin coating methods, the thickness of the thin film can be very precisely controlled. Thus, these methods are generally used in case that the thickness of the thin film is to be controlled, for example, on the submicron order. However, in case of a dipping method, both major surfaces of a substrate are normally coated with a thin film. Therefore, when only one major surface of a substrate is to be coated with a thin film, it is necessary to cover the other major surface with a masking tape or the like before the coating. In case of a spin coating method, it is possible to coat only one major surface of a substrate with a thin film having a well-controlled thickness. However, as a substrate is enlarged in size, it is necessary to prepare a larger device for holding and rotating the substrate. In case that a large-size substrate is used, it is necessary to very firmly attach the substrate to the rotating device for safety.
According to flow and direct roller coating methods, one major surface of a substrate is very easily coated with a thin film. However, in case of a flow coating method, a coating composition is flowed so as to make a curtain thereof above the substrate while the substrate is moved. This curtain has a certain width so as to cover the width of the substrate. Thus, as the substrate is enlarged in width, it is necessary to lengthen the curtain's width. With this, it is necessary to circulate a large amount of coating composition for reusing the same which has not been applied to the substrate. Thus, the solute concentration of the coating composition and its viscosity may increase in time. In case of a direct roller coating method, it may be difficult to adjust the contact degree between a roller and a substrate. Thus, as the substrate is enlarged in size, the thickness of thin film formed on the substrate may become uneven from portion to portion thereof. Therefore, this method may be inappropriate for allowing an optical thin film to have a thickness deviation of from -10% to +10% from the objective thickness. Furthermore, this method may be inappropriate for controlling the film thickness on the submicron order.
Furthermore, there is known a reverse roller coating method for applying a coating composition to a substrate. In this method, a reverse roller is rotated in a direction reverse to a moving direction of the substrate while the substrate is moved in the moving direction below the reverse roller. According to a conventional reverse roller coating method, it is possible to allow a thin film to have a uniform thickness of, for example, at least about 10 .mu.m. However, according to this method, it is difficult to allow an optical thin film to have a thickness of, for example, up to 1 .mu.m.
JP-A-5-57225 discloses a so-called squeeze type coating method with a roller coater. This coater has a pair or rollers 2 each having a rubber covering 2b. The rubber covering 2b is formed thereon with a groove 2c which is spiral about an longitudinal axis of the roller 2. While a coating liquid 14 is applied to a steel plate 1 by the roller coater, a shaped metal member 3 having an edge portion which conforms to the spiral groove 2c in shape is pressed against the roller 2 so as to uniformly abrade the spiral groove 2c. However, this method may not be suitable for forming an optical thin film on a substrate.
Thus, there is an increasing demand for a method of forming on a substrate an optical thin film which has a thickness of up to about 10 .mu.m (more particularly up to about 1 .mu.m) and a thickness deviation of from -10% to +10% from the objective thickness, without having the above-drawbacks.